1. Field of the Invention
The invention relates to microwave integrated circuit devices such as filters and delay lines. More particularly, the invention relates to such devices in which temperature compensation is provided.
2. Description of the Prior Art
In the design of on-board satellite repeater stations, differential QPSK (quaternary phase shift keyed) demodulation is frequently used as it provides the best compromise between carrier-to-noise performance requirements and on-board demodulator circuit complexity as carrier recovery is not needed. The successful implementation of direct modulation of differential QPSK signals requires the provision of a delay line which has very stable phase delay characteristics with expected temperature variations. Typically, a delay of 15-40 nanoseconds is required with an RF phase stability of no more than .+-.5.degree. for acceptable operation at frequencies on the order of 14 GHz.
The provision of an appropriate length of RF cable was the simplest prior art method of providing a required delay. However, in a communication satellite environment, the expected temperature change would cause the phase delay through a sufficiently long length of RF cable to vary by far more than would be acceptable for use in a differential QPSK demodulator without the use of a temperature-controlled oven. Moreover, the size and weight of the cable would ordinarily be excessive for use in such applications.
It has been known in the prior art to construct a filter device by locating stripline conductors upon a dielectric substrate with a ground plane underlying the dielectric substrate on the opposite side from the stripline conductors. A number of filter sections can be combined upon a single substrate to form a delay line filter using techniques well known in the prior art. Unfortunately, prior art microwave delay line devices exhibited very high temperature dependence upon the delay time of signals propagating through the delay line because of thermal expansion of the dielectric substrate and metal stripline conductors. Even using fused silica or quartz, one of the most temperature-insensitive RF substrate materials available, for an exemplary 10-pole Butterworth filter operating at 14 GHz, a phase shift in the output signal of 17.degree.-32.degree. for a 30.degree. C. temperature increment would be expected. Clearly, this variation is far too high for a differential QPSK demodulator in a communications satellite application.
A number of temperature compensation techniques have been known for compensating for various temperature-dependent properties in microwave integrated circuit devices. In one such one of these schemes as shown in U.S. Pat. No. 3,617,955 to Masland, a substrate material was used which has a negative dielectric temperature coefficient but a positive temperature coefficient of thermal expansion. The frequency shifts in one direction caused by thermal expansion were offset by the change of dielectric constant. This technique is not applicable at all frequencies because the magnitude and sign of the temperature coefficient of the dielectric constant is a function of frequency. Moreover, this technique will not provide the phase degree of stability required for use in a differential QPSK demodulator in a satellite repeater station over the expected range of temperature.
In other prior art temperature compensation schemes, stripline conductors were sandwiched between a dielectric layer and a contiguous overlying compensating layer. An example used in a resonator configuration is seen in U.S. Pat. No. 3,840,828 to Linn et al. Changes in filter parameters caused by thermal expansion and changes in the dielectric constant of the substrate were compensated for by changes in the physical properties of the overlying compensating layer. As in the previously-described technique, this arrangement was limited to lower frequencies and only to a relatively limited temperature range. None of the prior art techniques known were capable of being used in a filter application in which it was required that no dielectric material lie contiguous to the microstrip conductors in which there must be an open space between the surface of the substrate upon which the stripline conductors are located and an adjacent ground plane such as is required in microwave frequency applications.